Encroaching Swarm

At the beginning of your upkeep, amass Rats 1.
(Put a +1/+1 counter on an Army you control. It is
also a Rat. If you don't control an Army, create a 0/0
black Rat Army creature token first.)
At the beginning of your end step, return target
Rat creature card with mana value less than or
equal to the power of target Army creature
token you control from your graveyard to the battlefield.